<div class="doc-item" id="doc-748"><h2 class="doc-item-name">defonce<span class="namespace">(clojure.core)</span></h2><p class="macro">Macro</p><ul class="var-arglists"><li>(<span class="var-name">defonce</span> name expr)</li></ul><div class="var-docstring"><p>defs name to have the root value of the expr iff the named var has no root value,
  else expr is unevaluated</p></div><div class="var-source"><pre>; clojure/core.clj:3503
(defmacro defonce
  [name expr]
  `(let [v# (def ~name)]
     (when-not (.hasRoot v#)
       (def ~name ~expr))))</pre></div><div class="license"><p>Copyright (c) Rich Hickey. All rights reserved.</p><p>The use and distribution terms for this software are covered by the <a href="http://opensource.org/licenses/eclipse-1.0.php">Eclipse Public License 1.0</a>, which can be found in the file
          epl-v10.html at the root of this distribution. By using this software
          in any fashion, you are agreeing to be bound by the terms of this
          license. You must not remove this notice, or any other, from this
          software.</p></div></div>